Case Study Contributors
Barbara Ellis-Sugai, Forest Hydrologist, Siuslaw National Forest, Oregon
Location
Salmon River Basin, Siuslaw National Forest, Northwest Oregon, USA. MAP
Project Type
Pre-Project Conditions
Pre-Project Barrier
Velocity, depth, and leap barrier to all fish and other aquatic organisms
Hydrologic Characteristics
Ecological Value
Access to upstream habitat for all aquatic organisms, including:
Project Characteristics
Challenges
Project Contributors
Siuslaw National Forest
Project Funding
US Forest Service
Total Project Cost
| Engineering/Design | $ | 15,000 |
| Contract Inspection | $ | 10,000 |
| Construction/Materials | $ | 210,000 |
| Total | $ | 235,000 |
Completion Date
September 2004
Project Summary
The pre-existing crossing, dated 1930, was a concrete box culvert set perpendicular to the road and poorly aligned with Toketa Creek. The channel had been realigned to flows next to the road for nearly 130 ft (40 m) before becoming a meandering channel with a natural floodplain. The culvert was undersized, causing sediment deposition upstream and erosion of the road fill adjacent to the inlet. Large boulders had been added to the base of the roadfill to prevent additional erosion. The drop over the concrete outlet apron and the insufficient depth and excessive velocity made this culvert a barrier for fish passage.
Toketa Creek adjacent to the crossing has a gradient of 4.5%. The substrate consists of gravel and cobbles with sections of bedrock. Natural steps occur at spacing ranging from 5.3 ft (1.5 m) to 42 ft (12.8 m). The average spacing is 15 feet (4.6 m) and average step height is 0.4 ft (0.12 m).
The replacement structure is a concrete bottomless arch culvert spanning the 19 ft (5.8 m) bankfull channel width. The new culvert consists of sixteen 20 ft (6.1 m) wide x 5 ft (1.5m) long x 7 ft (2.1 m) high pre-cast concrete arches. The arches rest on cast-in-place concrete footings. To skew the culvert to the road and improve the alignment of the culvert with the stream, each section is individually skewed 45 degrees.
The simulated stream channel was built with boulder steps (¼ - ½ ton rock) spaced 15 ft (4.6 m) apart. Stream-simulation bed material was placed between each step at the finished design grade of 4.5%, with the expectation that the stream will scour and excavate the pools below each step. Shallow banks were built along the sides of the culvert next to the footings, with the thalweg in the middle.
Post Project Observations
Stream Adjustment Above and Below Culvert
A post-construction, pre-winter survey was done in September 2004, and repeated in 2005 and 2006. After construction, the drop at the outlet was eliminated, and the stream gradient was relatively smooth.
The thalweg profile measured after the first winter shows the steps emerging. Cross-sections at the inlet, outlet, and middle of the culvert show little change in streambed elevation.
The 2006 survey, done after a 5-year flow event, shows little change in the stream profile. The constructed steps inside the culvert are more visible but the channel bed below the outlet has aggraded slightly. The constructed stream banks remained relatively unchanged.
Lessons Learned
The streambed simulation in this culvert is working well, and blends with the streambed upstream and downstream of the culvert. The rock steps are working well as grade control.
Nearby, a replacement crossing with similar characteristics was built using randomly placed boulders to see if it would be as effective as constructed steps. They found that the randomly placed boulders didn’t work as well to stabilize and hold the streambed material in the culvert. During the first winter, a narrow channel was cut through the center of the crossing, and much of the streambed material was deposited downstream of the culvert. They concluded that rock steps are a better choice for grade control on streams with a gradient greater than 3%.
Published 09/27/07